Mine grey screens when I have my usb hub plugged in at startup (doesn't get to partition selection).
Android file transfer utility stops the keyboard and touchpad from working.
I've got a similar USB issue. I have a SATA HDD in an External USB 3.0 enclosure. When my 2011 iMac wakes from sleep, it needs to wait for the external drive to spin up. Major PITA.
Having similar weird USB issues on my 13" late 2013 MBP. It's been really, really crappy since I bought it. Same issues with keyboard and touchpad breaking under random applications. Apple seems to be completely ignoring the issue aside from a botched "fix" in November.